The invention relates to a method for electrical discharge machining according to the wire cutting principle. This method and the apparatus used with it can in particular be employed where extremely accurate cutting contours are required at the maximum working speed.
Of late, so-called short-duration pulse generators have been used as working pulse sources in wire cutting and have led to considerable advances with regard to the working speed (e.g. described in U.S. Patent Application Ser. No. 825,036) now U.S. Pat. No. 4,163,887, Buhler et al, assigned to the assignee of this application).
Although with this type of generator vibrations on the wire electrode can be eliminated, the greatly increased working forces cause a permanent deflection of the wire electrode, which is sensitive to bending. This deflection, which is in the direction opposite to the working direction, causes significant contour errors when cutting curved contours.
A temporary, uncontrolled deflection of the wire electrode may also be brought about by other forces, caused for example by gas bubbles in the dielectric, or turbulent flows thereof, or material stresses in the actual wire electrode.
Influencing of the cutting contour by deflection of the wire electrode must be prevented.
One approach to this problem is described in Japanese Pat. No. 119,393/74, which proposes a theoretical determination of the deflection by calculation and its compensation by means of a numerical control system. However, although this method may be satisfactory for straight cutting contours, serious problems occur in the case of a curved cutting contour because it is not possible to calculate the path of the deflection from one axial direction to the other. Therefore, this method is not practical.
Another approach to this problem is disclosed in DE-OS 2,635,766, to which U.S. Pat. No. 2,635,766 corresponds, which proposes reducing the generator output and the working speed in proportion to the increased curvature of the cutting contour. However, this has the disadvantage of very considerable cutting efficiency losses in the case of frequently curved cutting contours, while the error is only partly compensated.
Still another approach is proposed in GB Pat. No. 1,512,654, to which U.S. Pat. No. 4,104,502, assigned to the assignee of this application, corresponds, where the wire electrode is subjected to the action of additional currents and additional voltages in order to provide compensation by means of electro-magnetic and electro-static forces acting on the wire electrode. Unfortunately, this leads to additional loading of the wire electrode by electric heat and arc discharges, which once again brings about cutting efficiency losses. Furthermore, the electromagnetic forces are only active with ferromagnetic workpieces.
None of these three approaches is able to obviate faults caused by gas bubbles and turbulent flows in the dielectric or caused by material stresses in the wire electrode.